Edmund's "True Market Value" is the best I think. Also, look in the "Smart Shopper" forums for the "Real World Trade in Values" thread. It has postings from a real buyer of used cars and he is on the money with what cars are really worth.
2005 Nissan Sentra 1.8 SE, Code Red. '00 Tropic Green Ford Escort SE [Contour's little cousin]
